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Astrophysics

Beyond the stars

Light and shock waves warm up clouds that concentrate material formed by protons and neutrons

nasaThe galaxy NGC 3079, at the top, in which light and shock waves warm up the gas expelled from the nucleus, and the Crab Nebula, in which the shock heats up hydrogen (in green)nasa

Finally gaining force is an idea conceived a little more than two decades ago by two astronomers – the Brazilian Sueli Viegas and the Italian Marcella Contini – in order to explain chemical and physical phenomena observed in the entrails of gigantic clouds of gas and dust that permeate the galaxies and concentrate the greater part of baryonic material in the Universe, formed by protons, neutrons and electrons. The most recent evidence that Sueli and Marcella are right in what they say, with respect to the behavior of material in these obscure regions of the cosmos, comes from the observation of a peculiar type of galaxy: the galaxies with an active nucleus, so called because they concentrate almost all of their brightness in a small central region, the nucleus.

In collaboration with the astronomer Alberto Rodríguez Ardilla, from the National Astrophysics Laboratory, in Minas Gerais State, Sueli and Marcella analyzed the structure of the gas and dust cloud of the galaxy with active nucleus named Markarian 766, considered relatively close in cosmology terms: it is some 150 million light-years from the Milky Way – to have an idea of this distance, the light detected today by the astronomers left this galaxy some 150 million years ago. Discovered by the Armenian astronomer Benik Markarian in the 1960s, this galaxy shows an anatomy similar to that of the Milky way, within which the Solar System exists: it has a central region in the shape of a more luminous globe, surrounded by a fine disc of stars. In the heart of these galaxies a powerful black hole, with a mass millions of times greater than that of the Sun, swallows the matter in its surroundings and transforms it into energy, in part regurgitated back out into space in the form of light. Not very far from the devouring monster, these is a thick ring of gas and dust, home to recently born stars, which feed the interminable cycle of stellar life and death.

Sueli and Marcella managed to reconstruct the complete profile of light emitted by the central region of Markarian 766 – or as it is known Mrk 766 – starting from data obtained by the Hubble space telescope and by Rodríguez, using the telescope at the Mauna Kea Observatory, in Hawaii. Similar to the high and low traces of an electrocardiogram, this profile registers the quantity of light emitted by the galaxy and the cloud that surrounds it in different wavebands of the electromagnetic spectrum, from the least energetic such as radio waves to the highest energy such as X-rays. “It’s a type of colored signature that allows one to know the chemical composition of the galaxy and of the gas and dust cloud”, explains  Sueli, who at the end of 2005 completed a career of 30 years as an astronomer at the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of Sao Paulo (USP) and today lives in the United States with her husband – the Gary Steigman, from the Ohio State University -, and is dedicating herself to the science  diffusion.

In the light spectrum from Mrk 766 there was the evidence that Sueli and Marcella were so much looking for to prove the explanation that they had proposed long beforehand for the physical phenomena observed in the extra galactic clouds of gas and dust. Essentially formed by gases of the light chemical elements such as hydrogen, made up of only one proton and one electron, as well as heavier elements such as carbon and oxygen, these clouds impede the light from the nuclei of these galaxies from reaching the Earth, just like the mist of a cold morning that upsets the vision of the motorist on the highway. But the light blockage is not the only thing that occurs here.

The corpuscles of light (photons) from the galaxy’s nucleus transfer part of their energy to the gas and the dust particles, warming up the cloud – with the extra energy the atoms of hydrogen, silicon and carbon, among others, become electrically charges (ions) and emit light detected by telescopes in space and on land. In general, astronomers and physicists attribute the energy accumulated by these clouds only to this phenomenon of energy transference called photo-ionization. Sueli and Marcella, nevertheless, think differently.

“The energy transferred to the cloud only by photo-ionization could be some dozen to hundreds of times smaller than that which we observe”, says Sueli. Something else provides energy for these clouds to reach temperatures of some millions of degrees. For more or less 20 years Sueli and Marcella, from the University of Tel-Aviv, in Israel, have had a good idea of what this “something else” could be. The Brazilian astronomer was already distrustful that photo-ionization was insufficient to generate all of the energy of the extra galactic clouds when astronomer Marcella, a specialist in the phenomenon called shock, sought her out at the beginning of the  1980’s.

Together they developed a computer program that simulates the gas and dust cloud conditions called SUMA – soma in Italian, and is also the initials of SUeli and MArcella -, which added the photo-ionization to the effect of shock waves. In an era during which personal computers did not exist, much less laptops, they had to make do with what was then the most advanced at USP: a Burroughs computer, programmed by perforated cardboard cards. “The SUMA was a program so extensive that we had to have it working only at the weekends, otherwise the university would have stopped”, recalls  Sueli. Described in an article published in 1984 in Astronomy and Astrophysics, the SUMA is working today even in the more simple computers, as long as they have access to the internet.

How did they imagine that these two effects had been associated? Nothing very complicated. They knew that, to a certain degree, the nucleus light of these galaxies contributed to warming the gas and dust cloud. Also they knew that the cloud was not homogenous – and was a conglomerate of smaller clouds that displaced each other in a much less dense way. “These properties indicated that the chance of shock waves occurring in these regions was very high”, says Sueli. They had not imagined that the speed of displacement of these clouds was so high: in the case of the Mrk 766 galaxy, the clouds move at a velocity between 100 kilometers per second and 500 kilometers per second, as attested by  Sueli, Marcella and Rodríguez in an article in Monthly Notices of the Royal Astronomical Society of December 2005, one of the most important magazines in this area. “In the cloud region closest to the Mrk 766 nucleus the effect of photo-ionization predominates, whilst in the more distant region the main effect is caused by shock”, explains Sueli. To know more precisely the total energy of these clouds is essential for calculating physical properties such as temperature, density and the chemical composition of the gas of these regions – data that permits estimating the chemical evolution of galaxies and, in the ultimate instance, that of the Universe itself.

The proof that the shock and the photo-ionization work jointly is not restricted to the case of the Mrk 766. Sueli, Marcella and Rodríguez have noted similar results when analyzing another active galaxy, that of Ark 564. Who got closest to these results was a team headed by the astronomer Michael Dopita, from Australia, who invented a program that only takes into account the shock effect, but leaves aside the photo-ionization. In spite of having contestations to the interpretation of the two astronomers for the phenomenon observed with these clouds, astronomer Sueli moves forward confidently: “With the increase in the number of more precise observations of this central region of galaxies, the acceptance of our interpretation is becoming closer and closer”.

The Project
Evolution and activities of galaxies; Modality Thematic Project; Coordinator Sueli Viegas – USP; Investment R$ 2,247,008.35 (FAPESP)

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